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Difficulty: MediumLe Chatelier's Principle
Consider the reversible industrial synthesis of ammonia represented by the thermochemical equation:
N2(g)+3H2(g)⇌2NH3(g)ΔH=−92.4 kJ mol−1N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g) \quad \Delta H = -92.4 \text{ kJ mol}^{-1}
The addition of a finely divided iron catalyst to this system at equilibrium increases the rate of the forward reaction and thereby increases the final equilibrium yield of NH3(g)NH_3(g). Is this statement true or false?

Answer: Answer

Answer

False
The statement is false because a catalyst lowers the activation energy barrier by the same magnitude for both the forward and reverse reactions. As a result, both reaction rates increase by identical proportions, allowing equilibrium to be established more rapidly without shifting the equilibrium position or altering the yield of ammonia.

Step-by-Step Solution

1
Analyze the action of a catalyst on a reversible chemical reaction.
A catalyst provides an alternative reaction mechanism with a lower activation energy (EaE_a).
Lowering the activation energy allows a larger fraction of molecular collisions to be effective.
2
Evaluate the effect of the lower activation energy on both forward and reverse pathways.
The activation energy barrier is reduced by the same amount in both the forward and reverse directions.
Because both the forward rate constant (kfk_f) and reverse rate constant (krk_r) increase by the same factor, the equilibrium constant (Keq=kfkrK_{eq} = \frac{k_f}{k_r}) remains unchanged.
3
Determine the impact on equilibrium position and product yield.
The catalyst speeds up the attainment of equilibrium but has zero effect on the position of equilibrium or the final equilibrium yield of NH3(g)NH_3(g).
According to Le Chatelier's principle, equilibrium position is shifted only by changes in temperature, pressure, or concentration, whereas catalysts affect reaction rates only.

Key Concept

Effect of a catalyst on chemical equilibrium according to Le Chatelier's Principle
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Topics

Le Chatelier's Principle

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